Interpretive Summary: A channel catfish aquaculture system that uses biofloc bacteria to reduce levels of ammonia and nitrite in tank water was evaluated for the development of phytoplankton communities and common preharvest off-flavors. Although the compounds that cause “earthy” and “musty” off-flavors were detected in tank water, the levels of these compounds were low and resulted in less off-flavored catfish than might be encountered in pond-raised catfish in Mississippi

Technical Abstract:
The use of biofloc technology production systems continues to increase in the aquaculture industry worldwide. Recent research demonstrated that outdoor biofloc systems can be used to produce high yields of channel catfish (Ictalurus punctatus). However, studies have not yet been performed to determine the development and composition of phytoplankton communities and related off-flavor problems in these biofloc production systems. In this study, water samples were collected biweekly from May to November and channel catfish samples were collected during harvest in November from nine 18.6 m2 biofloc culture tanks. Water and fillet samples were analyzed for levels of the common off-flavor compounds geosmin and 2-methylisoborneol. The development and composition of phytoplankton communities in each culture tank was also monitored. In addition, water and biofloc samples were evaluated to assess the microbial sources of geosmin and 2-methylisoborneol within the culture tanks. Phytoplankton (including algae and cyanobacteria attached to bioflocs) biomass, as determined by concentrations of chlorophyll a in the water, gradually increased in all tanks over time. Phytoplankton communities that developed in the culture tanks were dominated by fast-growing, unicellular and small colonial types of green algae (chlorophytes) and diatoms (bacillariophytes) and slower growing, small colonial types of cyanobacteria (cyanophytes). Although geosmin and 2-methylisoborneol were present in the culture water of each tank during most of the study, levels were typically low and only one tank yielded catfish with geosmin and 2-methylisoborneol in their flesh at levels high enough to be designated as off-flavor. The microbial sources responsible for production of geosmin and 2-methylisoborneol in the culture tanks remain unknown.